The inventors have addressed the development of a non-invasive diagnostic technique using free DNA derived from a tumor in serum (Non-patent Documents 1 and 2). In recent years, the existence of microRNAs (miRNAs), non-coding RNAs that are not translated into proteins, has been confirmed. It has been reported that their expression profiles are tissue- or tumor-specific (Non-patent Documents 3 and 4). In pediatric cancers as well, it is reported in a study using cell lines (Non-patent Document 5) that expression profiles differ depending on the tumor. In rhabdomyosarcoma, which is the most common soft-tissue sarcoma in pediatric cancers, it is revealed that the expression of miRNAs specifically expressed in muscle is increased (Non-patent Documents 5 and 6). Additionally, it is also revealed that miRNAs of tumor origin exist in serum. Thus, the utility of miRNAs as biomarkers is suggested in colorectal cancer, lymphoma, prostatic cancer, liver cancer, and the like (Non-patent Documents 7-10).